This invention relates generally to feed processing equipment and more particularly to extrusion mills for pelletizing feed grains, formulas, and conditioned mixtures thereof.
Many feed compositions can be made into better quality pellets if more mechanical work is done to the feed on the face of the die before the feed is pressed through the pelleting holes of the die. Not only are the pellets produced under such conditions stronger, but they are also more thoroughly blended and conditioned as a result of the added mechanical work input.
One way of achieving this added work is to operate with a thick pad of feed between the die face and the rollers--usually one to three rollers per die. However, this condition can only be achieved if the rollers can be remotely adjusted while the mill is running and feed is being processed. Without such remote adjustment capability, the only way to operate with a thick pad of feed is to back-off the rolls, before starting the mill, to a gap which allows for the desired pad thickness and also accounts for thermal expansion during operation--a guess, at best. Still another reason for backing-off the rolls for mill start-up is to reduce the starting load on electric motor drives to extend motor life and to reduce power consumption. This would permit operation with smaller motors and lighter electrical service.
Early roller adjustments had to be made using a combination of jack screws, turnbuckles, and eccentrically mounted roller tires. For purposes of safety, these adjustments had to be made prior to starting the mill, and, since the rollers were not visible during operation, the adjustments had to be calculated and could not be verified when the mill was running. Also, starting the mill and the feed input with the rollers backed away from the die often resulted in development of an uneven pad and non-uniform pellet production.
To avoid these limitations, remote (external) roller adjustment systems have been developed with varying degrees of success in operation. The most common system uses single or double acting hydraulic cylinders in place of the jackscrews and turnbuckles to turn the eccentric shafts upon which the roller tires are mounted for adjustment. A single acting hydraulic cylinder can be monitored by measuring the volume of fluid going into or coming out of the cylinder to determine, ideally, the positions of the eccentric shafts. Leaks, however, can destroy the accuracy of such a method. Double acting cylinders can be used, but they take up more space and require more hydraulic lines. Generally they also require electronic position sensors within the mill which may fail due to the physically hostile environment.
At least one mechanical adjustment system employs disk cams mounted at the front and rear of the rollers, so that, when the cams are rotated, the cam action is applied to sliding blocks attached to the ends of the eccentric roller shafts. Cam position is controlled from outside the mill by means of a control shaft extending from the disk cams and having a "pointer" feature which can be used to indicate the size of the roller/die gap. The heavily loaded sliding blocks are precisely fitted in keyed ways which are subject to wear and corrosion and infiltration of feed material which may become compacted and cause jamming of the blocks. This may prevent roller adjustment and necessitate shut-down of the mill for cleaning and maintenance. Wear, on the other hand, may cause loosening of the blocks in the ways and permit roller shaking during operation. Such jamming and/or vibration can only be prevented by means of very complex sealing provisions for the blocks and ways. Because of the complexity of the sliding seals, deterioration may be rapid enough that the value of including such seals becomes marginal.
Existing cam type adjusters rely on springs to force the slide blocks against the cam surface. These springs have limited power and stroke. Thus, the slide blocks may not remain in contact with the cam, and, as a result, the rollers may not be backed off from the die and may become cocked.
Construction of mechanical adjustment systems must be very heavy and strong because of the high loading encountered when the rollers are moved outward toward the die while a feed material is being pelletized. As a result, space within the mill may become limited to an extent where the mill must be derated or remote roller adjustability may not be included.
The great diversity of animal foods and other feed materials available for pelletizing requires an adaptability of equal diversity from the pellet mill. In attempting to optimize the pelletizing operation for each of the diverse formulations encountered, a number of different operating conditions must be imposed. These difficulties, either alone or in combination and to varying degrees, are encountered in virtually every pelletizing operation because of variations in material properties, mill condition, and operating parameters.
The foregoing illustrates limitations known to exist in present feed extrusion pelleting mills. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.